A polyimide film is known as one of organic-based thin films used to manufacture a semiconductor device. The polyimide film is formed, for example, by dehydration condensation of two kinds of monomers. Specifically, as illustrated later in FIG. 2, a bifunctional acid anhydride, e.g., PMDA (C10H2O6: pyromellitic dianhydride), and a bifunctional amine, e.g., HMDA (C6H16N2: hexamethylenediamine) are used as the monomers. A polyimide film is formed by mixing the monomers in a solution to produce a polyamide acid solution that is a precursor solution and then by applying the precursor solution to a substrate such as a semiconductor wafer (hereinafter, referred to as a “wafer”) and simultaneously heating the wafer.
However, in integration of semiconductor devices electronic devices such as a cellular phone and the like. As such, the configuration has been suggested to stack a plurality of wafers each having such semiconductor devices formed thereon, and simultaneously, electrode portions of the respective devices are vertically connected to each other. The inventors have considered the following aspect in forming such three-dimensional configuration.
That is, for example, a hole-shaped concave portion is formed in such a manner that a device is formed on a surface of a wafer and dry etching is performed from the back side of the wafer so that an electrode portion on the underside of the device is exposed. Then, a conducting portion such as copper is buried in the concave portion. Simultaneously, disposing another wafer having a device formed thereon on the back side of the wafer, so that both the devices are electrically connected to each other through the conducting portion (specifically, including a bump and the like disposed between the wafers). Accordingly, a plurality of the wafers is sequentially stacked, thereby forming the integration structure of devices. Practically, although a process of forming the concave portion or a process of burying the conducting portion is performed while turning the wafer over, the surface of the wafer is here described as a side having the device formed thereon for convenience of illustration.
The aforementioned concave portion is formed to have a depth (e.g., 50 micrometers) until the device is reached from the back side of the wafer. Meanwhile, the concave portion is formed to have an opening dimension (diameter), e.g., a small diameter of 5 micrometers, so as not to interfere with a conducting portion of another integrated structure adjacent to the concave portion, i.e., so as to highly density the integrated structure as much as possible. Therefore, the concave portion has an extremely large aspect ratio (i.e., the ratio of the diameter to the depth of the opening).
In this case, since the wafer is made of silicon (Si), an insulation film is necessarily formed along an inner wall of the concave portion before the conducting portion is buried therein so that the wafer and the conducting portion are not electrically connected to each other through the inner wall of the concave portion (so that the wafer and the conducting portion are insulated from each other). Thus, the inventors attempt to apply the polyimide film as the insulation film. FIG. 12 shows the configuration described above, wherein reference numeral 1 designates a polyimide film, reference numeral 10 designates a concave portion, reference numeral 11 designates a device, reference numeral 14 designates a temporary fixing material, and reference numeral 15 designates a support substrate.
However, in the polymerization of the monomer, a large thermal energy of 30 kJ/mol or more is required to move monomer molecules. Specifically, the heating temperature of the wafer is set, for example, to 300 degrees C. or more. Therefore, such heating causes the dehydration condensation of the monomers on the wafer along the thickness direction of the polyimide film. Hence, the entire volume of the polyimide film is rapidly decreased, and thus, a large stress occurs in the wafer. Also, if a precursor solution is used, the precursor solution is buried in the concave portion, and therefore, the conducting portion cannot be formed. In order to obtain a high polymerization degree for the polyimide film (in order to increase the number of straight chains of imide molecules in the polyimide film), the mixture ratio of the respective monomers necessarily corresponds to 1:1. Therefore, since the concentration of each monomer should be strictly controlled, the configuration or control of an apparatus for forming the polyimide layer becomes complicated, and the kind (a combination) of available monomers is limited. In addition, the thickness of the polyimide film is changed depending on the concentration of the precursor (the concentration of each monomer) or the mixture ratio in the surface of the wafer.
Meanwhile, a method for forming a polyimide film of which the c-axis is oriented in the vertical direction with respect to a substrate by alternately depositing two kinds of monomers on a surface of the substrate is suggested. In this method, the temperature of the surface of the substrate is set to 45 to 50 degrees C., and the substrate is then heated to 300 degrees C. immediately after stopping the deposition of the monomers. However, according to the method, the concave portion as described above cannot be satisfactorily buried, and it is necessary to elevate the process temperature of the latter stage in which there is a high difference in temperature during a film forming process.